The present invention relates to a method and apparatus for making nonwoven fibrous webs that resist shrinkage when exposed to heat.
Typical melt spinning polymers, such as polyolefins, tend to be in a semi-crystalline state upon meltblown fiber extrusion (as measured by differential scanning calorimetry (DSC)). For polyolefins, this ordered state is due, in part, to a relatively high rate of crystallization and to the extensional polymer chains orientation in the extrudate. In meltblown extrusion, extensional orientation is accomplished with high velocity, heated air in the elongational field. Extending polymer chains from the preferred random coiled configuration and crystal formation imparts internal stresses to the polymer. Provided the polymer is above its glass transition temperature (Tg) these stresses will dissipate. For meltblown polyolefins, the dissipation of stresses occurs spontaneously since the polymer""s Tg is well below room temperature.
In contrast, some melt spinning polymers, such as polyethylene terephthalate (PET), tend to be in a nearly completely amorphous state upon meltblown fiber extrusion. This characteristic is attributable to a relatively low rate of crystallization, a relatively high melt temperature (Tm), and a Tg well above room temperature. The internal stresses from amorphous orientation within the elongational field are frozen-in due to rapid quenching of the melt, thus preventing relaxation which cannot be released until subsequent annealing above Tg. Annealing between Tg and the Tm for sufficient periods allows the polymer to both crystallize and dissipate internal stresses caused by elongational orientation. This stress dissipation manifests itself in the form of shrinkage that can approach values exceeding 50% of the web""s extruded dimensions.
The textile and film industries have successfully addressed dimensional instability in woven polyester fabrics and films using edge tentering during heatsetting or annealing. In edge tentering, the woven polyester fabric or film is held along its edges to a desired width as it passes trough an annealing oven. The heatsetting temperature ranges typically from about 177xc2x0 C. to about 246xc2x0 C.(350xc2x0 F. to about 475xc2x0 F.), and the dwell time ranges from about 30 seconds to several minutes. The annealed article is dimensionally stable up to the heatsetting temperature. While edge tentering is practical for films and woven fabrics, nonwoven fibrous webs typically lack sufficient tensile properties (i.e., fiber and web strength) to withstand conventional edge tentering procedures, resulting in a damaged web.
Various attempts have been made in the art to achieve a dimensionally stable polyester nonwoven fibrous web U.S. Pat. No. 3,823,210 (Hikaru Shii et al.) describes a method of manufacturing an oriented product of a synthetic crystalline polymer. The patent discloses drawing a crystalline polymer, applying tensile stress in the direction of the draw axis in a heated solvent, and under this condition extracting the soluble fractions of the drawn material.
U.S. Pat. No. 5,010,165 (Pruett et al.) describes a dimensionally stable polyester melt blown web achieved by treating a melt blown web composition with a solvent where the solvent has a certain solubility parameter, and drying the melt blown web composition.
U.S. Pat. No. 5,364,694 (Okada et al.) teaches that PET cannot give a meltblown web with small thermal shrinkage unless the melt-blowing operation is conducted at higher viscosity and with air under higher pressure than these melt-blowing conditions employed for other readily-crystalline polymers such as polypropylene. The patent teaches stable operation with high productivity is impossible under such strict conditions. The patent discloses that blending the PET with 2 to 25% of a polyolefin decreases the melt viscosity of the entire blend so that the polymer extrudates can be attenuated into fibers even by the comparatively weak force exerted by a low-pressure air of not more than 1.0 kg/cm2. The extruded polyolefin has a high crystallization rate. In the blend, the polyolefin forms minute islands in a continuous sea of PET. The multiplicity of crystallized polyolefin islands constitute restricting points that suppress movement of amorphous molecules of PET when the web is heated, thereby preventing the nonwoven fabric from shrinking to a large extent.
U.S. Pat. No. 5,609,808 (Joest et al.) describes a method of making a fleece or mat of filaments of a thermoplastic polymer having both a crystalline and an amorphous state. A melt-blowing head is operated under conditions to produce long filaments, which are collected on a sieve belt and form crossing welds at cross-over points. The resulting web is composed of filaments having a diameter of less than 100 micrometers and a degree of crystallinity of less than 45%. The web is heated to a stretching temperature of 80xc2x0 C. to 150xc2x0 C. and is then biaxially stretched by 100% to 400% before being thermally fixed at a higher temperature. The stretching station can have a downstream pair of rolls which are driven at a certain speed and an upstream pair of rolls driven at a higher speed to effect the longitudinal stretching. Transverse stretching is effected between pairs of diverging chains.
The present invention provides a method and apparatus for making a dimensionally stable or shrink-resistant nonwoven web of polymeric fibers. The resulting dimensionally stable, nonwoven fibrous webs can be used at higher temperatures with minimal change in fiber diameter, size, or physical properties as compared to conventional polyolefin webs. Nonwoven fibrous polyester webs dimensionally stabilized using the present method and apparatus are particularly useful as thermal and acoustical insulation.
The present method of making nonwoven fibrous webs does not require the use of additives that can have an undesirable impact on the base polymer properties. For example, polymer additives and polymer blends formulated to increase the dimensional stability of PET typically lower the melting point and glass transition temperature of the PET. This reduction in melting point and glass transition temperature negatively impacts on the use of PET for high temperature applications, such as automotive engine compartment noise attenuators.
In one embodiment, a nonwoven web of thermoplastic fibers is restrained on a tentering structure at a plurality of tentering points distributed across an interior portion of the web, rather than just along its edges. The nonwoven web is annealed while restrained on the tentering structure to form a nonwoven fibrous web, dimensionally stable up to at least the heatsetting temperature. The annealed nonwoven fibrous web is then removed from the tentering structure. In one embodiment, the tentering structure restrains the nonwoven fibrous web in a non-planar configuration during the annealing process.
The present invention also relates to a tentering structure for annealing nonwoven fibrous webs. The tentering structure includes a plurality of tentering points projecting distally from a tentering support. The tentering points can restrain the web in two or three dimensions.
As used herein,
xe2x80x9ccrystallization temperature (Tc)xe2x80x9d is the temperature where a polymer changes from an amorphous to a semicrystalline phase.
xe2x80x9cdimensionally stablexe2x80x9d refers to a nonwoven fibrous web that suffers preferably less than 20% shrinkage, more preferably less than 10% shrinkage, and most preferably less than 5% shrinkage, along its major surface when elevated to the temperature at which the nonwoven fibrous web was annealed.
xe2x80x9cglass transition temperature (Tg)xe2x80x9d is the temperature where a polymer changes to a viscous or rubbery condition from a glassy one.
xe2x80x9cheatsettingxe2x80x9d or xe2x80x9cannealingxe2x80x9d refers to a process of heating an article to a temperature greater than (Tg) for some period of time and cooling the article.
xe2x80x9cheatsetting temperaturexe2x80x9d refers to the maximum temperature at which the nonwoven fibrous webs are heated or annealed.
xe2x80x9cmelting point (Tm)xe2x80x9d is the temperature where the polymer transitions from a solid phase to a liquid phase.
xe2x80x9cnonwoven fibrous webxe2x80x9d refers to a textile structure produced by mechanically, chemically, and/or thermally bonding or interlocking polymeric fibers.
xe2x80x9cmicrofiberxe2x80x9d refers to fibers having an effective fiber diameter of less than 20 micrometers.
xe2x80x9cpercent crystallinityxe2x80x9d refers to the fraction of the polymer which possesses crystalline order. The crystalline fraction may include nearly perfect crystalline domains as well as domains possessing various levels of disorder, but yet be distinguishable from the lack of order present in an amorphous material.
xe2x80x9cpolymericxe2x80x9d means a material that is not inorganic and contains repeating units and includes polymers, copolymers, and oligomers.
xe2x80x9cstaple fiberxe2x80x9d refers to fibers cut to a defined length, typically in the range of about 0.64 centimeters to about 20.3 centimeters and an actual fiber diameter of at least 20 micrometers.
xe2x80x9ctentering pointxe2x80x9d refers to a discrete location where the nonwoven fibrous web is secured during annealing.
xe2x80x9cthermoplasticxe2x80x9d refers to a polymeric material that reversibly softens when exposed to heat.
xe2x80x9cultimate percent (%) crystallinityxe2x80x9d refers to the practical maximum achievable percent crystallinity for a material.